Electromagnetic spectrum classification by wavelength

Steps:

• Identify the given wavelength: 138 nm (nanometers).
• Recall standard wavelength ranges: radio waves (>1 mm), microwaves (1 mm–1 m), visible light (400–700 nm), X-rays (0.01–100 nm).
• Compare 138 nm to ranges: it falls between 0.01 nm and 100 nm.
• Conclude it belongs to the X-ray region.

Why D is correct:

• X-rays occupy wavelengths from about 0.01 nm to 100 nm, per standard electromagnetic spectrum divisions, placing 138 nm within this range.

Why the others are wrong:

• A. Radio waves have wavelengths longer than 1 mm (10^6 nm), much larger than 138 nm.
• B. Microwaves range from 1 mm to 1 m (10^6–10^9 nm), far longer than 138 nm.
• C. Visible light spans 400–700 nm, while 138 nm is significantly shorter.

Final answer: D